As we just heard, Muqtada al-Sadr was one of America's chief enemies during the war that followed the 2003 invasion. He made his name leading uprisings against U.S. forces in Baghdad and Najaf. The Pentagon once called his Mahdi Army the biggest threat to Iraq's security, which prompted us to wonder what American officials who worked on Iraq during those bloodiest years of war there - what they make of Sadr's apparent victory.

Zalmay Khalilzad was the U.S. ambassador to Baghdad from 2005 to 2007. And he joins me now from his office in Arlington in northern Virginia. Ambassador, welcome.

ZALMAY KHALILZAD: Well, it's great to be with you.

KELLY: Good to have you with us. I want to ask how surprised you are. A lot of the headlines today noting Muqtada al-Sadr as surprise front-runner and the, quote, "surprise lead he's taken." How surprised are you by how this appears to be shaping up?

KHALILZAD: Well, I am surprised. I thought that Prime Minister Abadi, who had led the Iraqis in the war against ISIS, would do better. But as we learned with Churchill after World War II and with President Bush Sr. after the Gulf War, the voters after a war prioritize differently. And Muqtada and his group, which is an unusual alliance with communists and liberals, has emerged on top with perhaps 54 or so seats.

KELLY: Right. He has refashioned himself, reinvented himself as a crusader against corruption. And I want to ask you, do you believe this is a sincere transformation?

KHALILZAD: Well, it is possible he has evolved. He was against the political process, at one point resorted to military force. We fought him. He thought the U.S. occupation was a mistake, that we had said we had come for liberation and we ended up occupying Iraq. And now embracing the political process and participating in it at one level can be seen as a success because if someone who was a warrior now becomes a politician, that is itself a positive development.

KELLY: But this - he wasn't just a warrior. He has blood on his hands for the deaths of U.S. soldiers, many U.S. soldiers. How do you reconcile that person with a man who may now hold the reins of power in Iraq?

KHALILZAD: Well, of course you're right about that. He does have the blood of American soldiers on his hand. And we killed a lot of his people. We were on the verge of killing him when Ayatollah al-Sistani intervened. And...

KELLY: Ayatollah al-Sistani, the most senior Shiite cleric in Iraq.

KHALILZAD: Indeed. And arranged for him, for Muqtada, to leave Iraq and go to Iran for a while. But now he says he has changed. He is improving relations with the Middle Eastern countries. He's participating in the political process. He's even saying politicians should not be in government, they should control the government from Parliament, that technocrats should address the issues of concern to the people. So perhaps he is on a journey from becoming a militant fighter to becoming a political leader. And if that truly happens, it will be positive because that's the definition of success - to turn enemies into constructive partners.

KELLY: You sound, Ambassador Khalilzad, cautiously optimistic about the way this vote appears to be turning out. And I guess that surprises me a little bit given this is not an outcome the U.S. would have predicted, certainly not one that the U.S. would have hoped for back in the days when you were ambassador to Baghdad.

KHALILZAD: Well, I would have - in the days that I was ambassador would have liked to see Muqtada become what he has become, for his forces to abandon war and for his forces to join politics and for politics to break out.

KELLY: I want to ask you to take the long view. This is a big question. But as you look back at what the U.S. set out to achieve with the invasion in 2003 and you look today at where Iraq is 15 years later, does it feel worth it?

KHALILZAD: Well, of course if we knew then what we know now about the weapons of mass destruction, of course not. But given that mistake that we made, Iraq is in a more promising moment at the present time. But they have a long way to go and face many challenges both internally and regionally. And we will have to see whether the new leadership, the new government that has to be formed - and I hope it will be formed quickly - will be able to overcome these challenges successfully.

KELLY: Briefly, what do you see as the top of the list of Iraq's challenges?

KHALILZAD: I think bringing the Iraqis together, that will be the biggest challenge. Without that, sectarianism returns or ethnic conflict returns. But if they can overcome that challenge of sectarianism and ethnic conflict, then Iraq can have a bright future. Although the challenges will take time to deal with, but it'll be on the right path.

KELLY: That is former U.S. Ambassador to Iraq Zalmay Khalilzad. Ambassador, good to speak with you.

KHALILZAD: It's great to be with you. Transcript provided by NPR, Copyright NPR.